A high-k metal gate fin field-effect transistor (HKMG FinFET), as shown in FIG. 1, may include a covering layer 110, a P-type work function metal layer 120, an N-type work function metal layer 130, and a barrier layer 140. A gate voltage threshold may be adjusted by adjusting a work function of a metal gate, thereby achieving multiple voltage threshold values.
Usually, the work function of the metal gate may be adjusted by changing the thickness of the metal gate, for example, by adjusting the thicknesses of titanium nitride, tantalum nitride, titanium aluminum carbide, or a compound thereof. For the HKMG FinFET, as the length of the gate is shortened, the space available for changing the thickness of a metal layer is limited, especially for a P-type low-threshold device. Therefore, new optimization solutions are desirable for a metal gate FinFET.